The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the correction of motion artifacts in MR images.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles, or xe2x80x9cviews,xe2x80x9d in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Depending on the particular view order used, object motion during the acquisition of NMR image data can produce blurring or xe2x80x9cghostsxe2x80x9d in the phase-encoded direction, or both. For most physiological motion each view of the NMR signal is acquired in a period short enough that the object may be considered stationary during the acquisition window. In such case the blurring and ghosting is due to the inconsistent appearance of the object from view to view. Motion that changes the appearance between views such as that produced by a patient moving, by the respiration or the cardiac cycle, or by peristalsis, is referred to hereinafter as xe2x80x9cview-to-view motionxe2x80x9d. Motion may also change the amplitude and phase of the NMR signal as it evolves during the pulse sequence and such motion is referred to hereinafter as xe2x80x9cin-view motionxe2x80x9d.
One method for reducing motion artifacts in NMR images is referred to in the art as xe2x80x9cgradient moment nullingxe2x80x9d. This method requires the addition of gradient pulses to the pulse sequence which cancel, or null, the effect on the NMR signal phase caused by spins moving in the gradients employed for position encoding. Such a solution is disclosed, for example, in U.S. Pat. No. 4,731,583 entitled xe2x80x9cMethod For Reduction of NMR Image Artifacts Due To Flowing Nuclei By Gradient Moment Nullingxe2x80x9d.
A successful method for correcting MR images for motion artifacts employs navigator signals acquired during the scan. As described in U.S. Pat. No. 4,937,526, such navigator signals are acquired periodically during the scan, and the information in these signals may be used to correct the image data for patient motion. This method is also disclosed in U.S. Pat. Nos. 5,581,184; 6,118,273; 5,800,354; 6,076,006 and 5,539,312. Unfortunately, acquisition of the navigator signals increases the scan time and can disturb the steady-state magnetization established during fast scanning.
An automatic correction method has been proposed in International PCT application WO9801828A1 in which the entropy of the reconstructed image is examined as a focus criterion by which to iteratively adjust motion estimate. The clinical application of the autocorrection method has been made possible by the discovery of improved metrics used to evaluate the quality of the image during each iteration. A number of such metrics are disclosed in co-pending PCT patent application No. PCT/US99/08123 filed on Apr. 14, 1999 and entitled xe2x80x9cAutocorrection of MR Images for Motion Artifacts.xe2x80x9d One disadvantage of this correction technique is that it requires many repeated iterations and is computationally intensive.
The need for better motion correction methods has increased with the use of MRI systems having higher field strengths (e.g. 3 Tesla). Such systems enable the acquisition of images with smaller voxel sizes (e.g. smaller than 1 mmxc3x971 mmxc3x971 mm), with resulting higher image resolutions. However, such high resolution images are also easier to corrupt with motion artifacts.
The present invention is a method for correcting acquired NMR image data for subject motion, and particularly, a correction method that does not require the addition of navigator pulse sequences to the scan. The method includes acquiring a k-space NMR data set by repeating an NMR image pulse sequence to sample k-space as a series of k-space segments; reconstructing a series of tracking images from the corresponding series of k-space segments; calculating the movement of a point object in the subject during the acquisition of each k-space segment by detecting the movement of a corresponding point spread object in each tracking image; calculating a corrective phase shift for each k-space segment based on the detected movement during the acquisition; and reconstructing an image using the corrected k-space segments. The k-space segments are selected such that the location of the point object can be determined from the point spread object in their reconstructed tracking images, and in the preferred embodiment, the elliptical centric k-space view order is used for a three-dimensional acquisition, and each k-space segment is an annular region (i.e. ring) that surrounds the origin of k-space.
A general object of the invention is to correct acquired NMR image data for bulk (i.e. rigid body) subject motion without the need for additional, interleaved navigator pulse sequences and without the need for computationally intensive, iterative auto-correction post processing. By judiciously selecting the view order, tracking images can be reconstructed which enable segments of acquired k-space data to be separately motion corrected. A point object that produces an intense NMR signal is located in the field of view and produces a point spread object in each reconstructed tracking image. The point object may be a fiducial which is attached to or inserted in the subject being imaged, or it may be part of the subject, such as a blood vessel imaged in cross-section, containing contrast agent. Annular regions (rings) of k-space are used in the preferred embodiment since, with the elliptical centric view order, the views acquired during any time interval At will lie within such a ring. Thus, the entire acquisition can be sub-divided into a series of time intervals, corresponding to a series of concentric rings.
A more specific object of the invention is to motion correct a contrast-enhanced magnetic resonance angiogram (CE-MRA). The flow of contrast agent through the vasculature of interest provides intense point objects which move with the patient . A centric view order, preferably elliptical centric is used to acquire the NMR image data when the contrast agent bolus arrives in the vasculature of interest. The main advantage of using the elliptical centric view order for CE-MRA is that it can provide a high degree of venous suppression. The view order is determined by sorting the views according to their distance to the center (origin) of the ky-kz plane. The sampling trajectory starts at the center of k-space and spirals outward. Each xe2x80x9cringxe2x80x9d of k-space sampled in this spiral trajectory may form a k-space segment which can be separately motion corrected using the present invention.
The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims herein for interpreting the scope of the invention.